The present invention relates to binary data storage and retrieval and, more particularly, to a memory element for a system in which data is written and/or read by a scanning electron beam.
In the Journal of Applied Physics, Vol. 47, No. 5, May 1976, at page 1741, there appears an article entitled "Storage and examination of high-resolution charge images in Teflon foils" by J. Feder. The headnote to the article states that high-resolution charge patterns at various charge densities were written on Teflon TFE foils using a 10-kV electron beam with a 1/2 micrometer spot diameter and that resolutions of 50-100 line pairs/mm were observed by examination with an electron beam and use of xerographic development techniques. It concluded that charge spreading in the Teflon and the examination processes were factors limiting resolution and observed that development of stored patterns up to three months after their writing showed that resolutions of 100 line pairs can be maintained for this period.
According to the Feder article, "Teflon foils are capable of storing charges for time periods on the order of decades." The introduction suggests that "in addition to being of interest in xerography, the methods have possible application in the storage/retrieval of digital and pictorial information." However, no example deals with digital information. The object of the investigation the results of which are summarized in the article was to develop techniques for examining high-resolution charge patterns.
All charge patterns were written on 25 micrometer Teflon TFE (polytetrafluoroethyene) foils at Bell Laboratories using a Bell Laboratories Electron Beam Exposure System. The test pattern used consisted of 14 repetitions at different scales of a basic L configuration of lines of specific linewidth at various orientations, plus filled regions containing spaces of the particular linewidth. The linewidths ranged from 1 to 30 micrometers. The entire test pattern had a dimension of approximately 3.1.times.3.3 mm. Samples were prepared by stretching metalized foils of 25 micrometers Teflon TFE across chrome-coated 6.35-cm square glass plates with the metalized surface of the foil in contact with the chromed side of the glass.
The discussion of the results presented in the Feder article are of considerable interest. It is stated that both electron beam and electrophoretically examined patterns show a spreading of written areas. Moreover, it is stated that "images obtained in electron beam examination appeared somewhat fuzzy and defocussed (probably due to the low read beam energy) and were marred by background noise."
Speculating upon the causes for the limited resolution, the article observes that the electron beam machine used in the writing process provides a reasonably good 1/2 micrometer electron spot and is capable of repeated beam positioning within 1/2 micrometer in a specified focal plane. Based upon the 100 line pairs/mm resolutions that were achieved, the article estimates that a recording density of 10.sup.10 bits/m.sup.2 (10.sup.6 bits/cm.sup.2) was obtained, assuming a density of 1 bit per 10 micrometer square section of the Teflon.